As compared with widely used amorphous silicon (a-Si), an amorphous (noncrystalline) oxide semiconductor has high carrier mobility, a high optical band gap, and film formability at low temperature and, therefore, has been highly expected to be applied for next generation displays which are required to have a large size, high resolution, and high-speed drive, resin substrates which has low heat resistance, and the like.
Of oxide semiconductors, an amorphous oxide semiconductor containing indium, gallium, zinc, and oxygen (In—Ga—Zn—O, hereinafter also referred to as “IGZO”), which has a considerably high carrier mobility, is particularly preferably used. For example, Non-Patent Documents 1 and 2 disclose a thin-film transistor (TFT) including a thin oxide semiconductor film of In:Ga:Zn=1.1:1.1:0.9 (atomic % ratio) as a semiconductor layer (active layer). Patent Document 1 also discloses an amorphous oxide containing elements, such as In, Zn, Sn, Ga, and the like, and Mo, where Mo has an atomic composition ratio of 0.1 to 5 atomic % with respect to the total number of metal atoms in the amorphous oxide. A TFT including an active layer of IGZO doped with Mo is disclosed in the example of Patent Document 1.